1. Field of the Invention
The present invention relates to pumping equipment. More particularly, the invention relates to, in one embodiment, a production system for producing fluids from a well that includes a jet pump and a submersible pump.
2. Description of the Related Art
The information described below is not admitted to be prior art by virtue of its inclusion in this Background section.
As the technology for offshore deep-water exploitation becomes available at a reasonable cost, the number of sub-sea completions in deep and ultra-deep waters is expected to increase significantly. Evidence of this expected increase in deep-water production may be seen in the large number of tracts in water deeper than 5000 feet that were leased in the recently completed Gulf of Mexico Outer Continental Shelf (OCS) lease sales. FIG. 1 presents the evolution of the tracts receiving bids in the recent lease sales and clearly emphasizes the expectations of the oil companies. (The asterisk in FIG. 1 denotes years in which a royalty relief program was in effect.) FIG. 2 presents a bar graph that shows the increasing production (expressed in % increase by year) from the deep and ultra-deep waters in the Gulf of Mexico. Other important evidence of increased deep-water production may be seen in the increasing activity of Petrobras. The Brazilian Petroleum Company started the exploitation of the Campos Basin offshore Rio de Janeiro, Brazil, in 1979 at water depths of 300 feet, and since then has worked continuously towards deeper waters where some important discoveries have been made. Today, high productivity wells have been producing steadily and successfully at water depths greater than 5000 feet in the Marlin and Albacore fields.
Production platforms are typically installed when producing from offshore wells. While the installation of a production platform in deep water is sometimes technically feasible, such an installation is more complicated, and thus more expensive, than installing a production platform in shallower water.
Consequently, the host production platforms in offshore petroleum production projects are usually installed in shallow water, which often requires a long flowline between the platform and the deep wells. With the wells located far from the host platform, the wellhead flowing pressures generally have to be maintained at a level sufficient to overcome high frictional losses plus the hydraulic head for the produced fluids to be able to flow back to the platform. The high wellhead pressure required to flow production back to the host platform will in turn tend to limit a pressure differential (or drawdown) that may be established at the reservoir. As a result, the production rates of the deep wells may be reduced to uneconomic levels.
A possible solution to the problem created by the installation of the host platform far from the production wells is the application of existing artificial-lift (AL) methods. AL methods supply the fluids produced from the well with sufficient energy to generate adequate drawdown at the formation while maintaining a high enough wellhead pressure to transport the fluids to the host platform at a desired flow rate. The AL method most commonly used for sub-sea offshore petroleum production is the gas lift (GL). A purpose of the GL method is to inject gas into the tubing string downhole in order to reduce the hydraulic head without increasing the friction losses so that the net result is an increase in the wellhead pressure for a fixed bottomhole pressure.
While the increase in the gas-liquid ratio (GLR) obtained with the GL method is highly beneficial for vertical multiphase flow, such an increase is not as helpful for horizontal flow. For long-distance horizontal multiphase flow, the net result of the increase in the GLR may be detrimental since the friction loss increases and there is little or no reduction in the hydraulic head. In addition, the increased GLR will create an operational problem with long-distance horizontal flow due to the instability of the slug flow that is expected to occur. Another problem with the GL method is that it requires an annulus lift-gas line, which for long distances will significantly increase the final cost of the project.
Pumping AL methods are also available for sub-sea applications. Such methods include the electrical submersible pump (ESP), the Progressing Cavity Pump (PCP), and the Jet Pump (JP). Present technology applies ESP""s for pumping of liquid with small amounts of free gas (up to about 5% or so) while JP""s are used to pump liquids using a liquid power fluid. An ESP typically includes a multistage centrifugal pump driven by a coupled electric motor. The pump may be installed inside the well at the end of the tubing string, and is typically situated at a certain depth below the fluid level. An electric cable connecting the surface transformer to the electric motor feeds electric power.
The JP is an AL method with no moving parts. The JP, which primarily consists of a body with a nozzle, a throat, and a diffuser, is set in a nipple inside the tubing string. Substantially clean power fluid is pumped down from the surface to the pump through the tubing. This power fluid passes through the nozzle, creating a low-pressure region connected to the pump intake so that the well fluid is suctioned into the throat region of the JP. The mixed fluid, i.e., power fluid plus produced fluids, exits the pump through the diffuser into the casing with sufficient head to overcome the hydraulic head plus the head losses.
To date, the majority of the pumping AL systems are being operated at conditions where there is a minimum of free gas present at the pump intake. As may be learned from Ref. 1 to Ref. 6, that free gas is, in many instances, detrimental to proper operation of these pumps. Consequently, it is not recommended to apply these systems without some provision for separation of the gas before reaching the pump intake. A requirement for the application of AL methods to sub-sea petroleum production is the necessity to operate relatively efficiently with a multiphase gas-liquid mixture because it is not desired, for economic reasons, to have an extra produced-gas flowline for each well. Without the annulus flowline, it is generally not possible to utilize the annular space as a downhole separator and to vent the gas at the casinghead.
The application of an ESP to a well having a high free-gas volume at the pump intake usually requires the installation of a gas separator. The use of a gas separator, however, may require the costly installation of an extra flow line to vent the separated gas to the host platform. Therefore, it would be desirable, in some embodiments, to design a production system that would allow an ESP to be used in a high free-gas well without requiring the installation of a vent line.
A production system may include a submersible pump and a jet pump. The submersible pump may be arranged within the well. The jet pump may be arranged within the well downstream of the submersible pump. The jet pump may include a power fluid intake configured to receive a power fluid and a produced fluid intake configured to receive a produced fluid. The power fluid intake may be in fluid communication with the submersible pump. The produced fluid intake may be in fluid communication with gas within the well. In an embodiment, the produced fluid intake may be in fluid communication with separated gas within an annulus of the well. The system may allow, among other things, a submersible pump (possibly an ESP) to be used in high GLR wells without installing a gas vent line.
In an embodiment, the jet pump may be positioned at the discharge of the submersible pump, and may use the fluid pumped by the submersible pump as a power fluid. In addition, a gas separator may be positioned upstream of the submersible pump. The gas separator may be configured to separate gas from liquid to produce separated gas and separated liquid. The separated liquid may be drawn into the submersible pump, while the separated gas may be segregated downstream within the annulus. The jet pump may then draw in the separated gas through the produced fluid intake, and later compress the gas and entrain the gas back into the separated fluid stream to be pumped to the surface. The use of a gas separator may reduce the amount of free gas that the submersible pump ingests and, as a consequence, may increase the performance of the submersible pump. Such a production system may be especially useful for wells with high GLR.
The system may allow a submersible pump and a jet pump to be combined into a single integrated system having the objective of economically producing a well without reducing the efficiency of the submersible pump or increasing the cost of the installation. The application of the system may increase the number of satellite wells that are able to use artificial lift to increase or maintain oil and gas flow rates, since high GLR wells may be produced using the system. Application of the production system may increase the profitability of future exploitation projects because it may be possible to increase the distance between the host platforms and the wells, which may result in a reduction of the number of host platforms needed. This new technology may be applied to any petroleum production well, but may have particular use in deep-water offshore exploitation.
Therefore, certain embodiments of the present production system may have one or more advantages. The system may provide an efficient artificial lift method for offshore and land (i.e., onshore) wells where the gas to oil ratio has increased past the operating limits of ESPs. Further, the system may provide an artificial lift method for deep offshore sub-sea wells without the need for a separate sub-sea gas vent line. The system may reduce power requirements for conventional ESP installations by reducing the required discharge pressure. The system may increase production rate by reducing the flowing bottom hole pressure in ESP wells. In addition, all elements of the downhole production system may be installed at once or at different times in the life of the well or wells being produced.
In many conventional ESP installations on integrated offshore platforms and onshore installations, gas from a reservoir is permitted to escape from the bottomhole fluids prior to its entering the submersible pump. This gas may be produced up the annulus as casinghead gas, and may be removed separately from the well at the casinghead, from which it may be directed into a separate pipeline from the produced fluids or vented. Because of the expense of a separate flow line and the environmental and/or safety concerns of venting, it may be beneficial to provide a way to produce these gases.
In an embodiment, the present production system may be used to produce such a well. That is, the production system may further include a casinghead valve configured to selectively permit gas within the annulus to pass into a conduit outside of the well. The conduit may be connected to a pipeline to be transported to a production facility, or to a vent. The casinghead valve may initially be open to permit casinghead gas to pass into the conduit. Subsequently, the casinghead valve may be closed to substantially prevent gas within an annulus of the well from escaping. Pressure within the annulus may be allowed to increase to a pre-determined pressure before initiating pumping of well fluids with the submersible pump. Once normal operation of the submersible pump and jet pump begins, the casinghead gas may be suctioned into the produced fluid intake of the jet pump, compressed, and entrained with the produced fluids pumped into the power fluid intake of the jet pump from the submersible pump.
Furthermore, an embodiment of a production system may be a packerless (i.e., open annulus) completion. That is, the annulus of the well defined between the production tubing string and the casing string may be devoid of isolation packers. It may be beneficial, however, to use isolation packers with wells, and thus the present production system may be used with such devices. Therefore, an embodiment of the production system includes an isolation packer positioned within an annulus of the well. The isolation packer may be positioned downstream of the jet pump and between a tubing string and a casing string within the well. The isolation packer may be used to trap well fluids and gases downhole of the packer. This configuration may reduce the pressure in the annulus gas with a corresponding decrease in flowing bottomhole pressure. Such a design may allow for the production of the well at higher rates if the pressure within the annulus upstream (e.g., downhole) of the packer is maintained above bubble point pressure.
In addition, an embodiment of a production system may combine a production system including a jet pump and a submersible pump with gas lift injection techniques. As noted above, gas lift is an artificial lift method in which gas is injected into the production tubing to reduce the fluid gradient of the fluids being produced. Gas lift processes may reduce the flowing bottomhole pressure, and thus the submersible pump discharge pressure and power requirements.
In an embodiment, the production system may include a gas lift injection system configured to inject gas within the well. The gas lift injection system may be further configured to inject gas into an annulus of the well. In such a configuration, the jet pump may be used as a substitute for the operating gas lift valve of a conventional gas lift injection assembly. Thus, gas injected into an annulus from the gas lift injection system may enter a tubing string within the well through the produced fluid intake of the jet pump to supply gas lift forces on fluids within the tubing, thereby reducing the flowing bottomhole pressure.
An embodiment of the production system may include at least one, and possibly a plurality of, gas lift valve(s) arranged downstream of the jet pump. The gas lift valves may further be arranged along the tubing string uphole of the jet pump. The gas lift valves may each be configured to selectively permit gas injected into the annulus to pass therethrough and, in an embodiment, to pass through the gas lift valves into the tubing string. That is, the gas lift valves may be configured to open and close to permit and prevent, respectively, fluids from passing therethrough under certain pre-determined conditions.
In another embodiment, the gas lift valves may be unloading gas lift valves. Thus, the gas lift valves may be used to unload liquid from the well to allow gas to be injected into the produced fluid intake of the jet pump. In such an unloading process, the fluid level may be above at least one, and possibly all, of the gas lift valves within the well. Gas may then be injected into the annulus of the well to depress the fluid level therein. As the fluid level within the well drops below each gas lift valve, the gas lift valves may each selectively permit gas injected into the annulus to enter the tubing, further aiding in the depression of the well fluid level. After injected gas has been selectively permitted to pass through each of the gas lift valves, the fluid level within the well may be lowered below the jet pump. The gas lift valves may remain closed when the fluid level within the well is below the jet pump (e.g., during normal operation of the production system), allowing substantially most or all of the injected gas to enter the tubing string through the jet pump.
Advantageously, gas injection into a jet pump as presented herein may allow for lower gas lift injection pressures or injection of gas at higher rates. In either case, the efficiency of such a system may be significantly improved over conventional gas lift installations.